The invention is directed to a process for the pretreatment of stainless steel, especially chrome-nickel steels for a direct electrolytic (galvanic) gold plating with strongly acid baths.
The electrolytic deposition of good adhering metal coatings on rust free, highly alloyed steels always has presented great difficulties through the difficult to remove impervious passive layer. Determinative for the passive behavior of these steels is a quickly forming coating layer, preponderantly of Cr.sub.2 O.sub.3, a very difficultly soluble oxide. For this reason for the preparation of the stainless steel there must be carried out before the electrolysis a removal of the passive layer by etching in corrosive mineral acids, partially under the influence of a current. Thus for examples it is recommended to use for this purpose 10-20 vol. % of concentrated HNO.sub.3 with 1-2 vol. % of concentrated H.sub.2 F.sub.2 at 50.degree. C., or to employ H.sub.2 SO.sub.4, CrO.sub.3 and H.sub.2 F.sub.2 at room temperature, or to activate the parts cathodically in various acids. However, almost all processes attack the base material which is undesired particularly with high gloss polished parts. There are further problems with these known processes if there are present stainless steel objects with parts, e.g. of German silver or copper, worked into them. Almost all of these processes also cause considerable corrosion problems in the electrolytic plants.
There are also known activating processes with deposition of an intermediate layer, usually nickel, from a strong hydrochloric acid solution. However, these likewise have distinct disadvantages. Besides corrosion problems in using these baths the so activated and subsequently gold plated parts are substantially more susceptible to corrosion than directly gold plated parts. Starting with faults the nickel intermediate layer corrodes until there is complete separation of the gold superimposed layer. With jewelry parts that are worn directly on the skin a nickel under layer is undesired because of allergic reactions of individual people and perhaps cancerogenic action.
To avoid the undercoating with nickel there have been attempts to activate stainless steel parts directly in the gold bath. Fletcher U.S. Pat. No. 4,168,214 describes a gold bath based on a "dilute aqua regia" which is in a position to activate stainless steel directly in the bath. However, the bath likewise has the disadvantage that it attacks the surface, especially at soldered places. Besides the bath is bad to handle since the corrosiveness increases with time of operation. (The entire disclosure of Fletcher is hereby incorporated by reference and relied upon.) In only a few cases other known strongly acid gold baths for direct coating of stainless steel result in a sufficient adhesion of the gold layer. If the steel part to be coated, for example watch bands, was subjected to for example tempering, soldering, polishing with development of heat, treating in passivating dips or electropolishing before a treatment would be applied underneath which permits formulation of a strongly passive layer, there could not be produced sufficient adhesiveness through direct gold plating.
Therefore, it was the problem of the present invention to develop a process for the pretreatment of stainless steel for a direct electrolytic (galvanic) gold plating in a strongly acid gold bath (e.g. a bath on the basis of KAu (CN/x) containing a mineral acid like phosphoric-or sulfuric acid) which attacks the metal surface as little as possible and produces a good adhesiveness of the deposited gold coatings.